1. Field of the Invention
The present invention relates to a low melting point glass composition suitable for covering and insulating the electrodes for plasma display, and a substrate for a plasma display panel (PDP).
2. Discussion of Background
In recent years, PDP has attracted an attention as a thin flat plate type color display device of large size. In such a display device, thin line electrodes are formed on a glass substrate to let picture elements for an image produce plasma discharge. To secure insulation between these electrodes, to produce a plasma under a stabilized condition and to prevent these electrodes from erosion by the plasma, these electrodes are covered with an insulting glass layer. This glass layer is required to have a thermal expansion characteristic which satisfies a predetermined relation with the glass substrate, to avoid cracking or breakage during the process for preparation of the panel or during the display operation.
Heretofore, various types have been proposed for an insulating glass layer to be used for such covering. JP-A-47-4318 (U.S. patent application Ser. No. 61842, filed Aug. 6, 1970), JP-A-47-6318 (U.S. Pat. Nos. 3,989,877 and 3,634,791), JP-A-47-6319 (U.S. Pat. Nos. 3,918,136, 3,976,823 and 3,896,323) and JP-A-47-7275 (U.S. Pat. Nos. 3,863,089 and 3,846,171) disclose a composition comprising 73.3 wt % of PbO, 13.4 wt % of B.sub.2 O.sub.3, and 13.3 wt % of SiO.sub.2.
JP-A-48-13080 proposes a low melting point glass containing boric acid and lithium oxide and not containing lead.
JP-A-50-23414 proposes a glass composition comprising PbO, B.sub.2 O.sub.3, SiO.sub.2 and Al.sub.2 O.sub.3 and further contains at least one of CaO and MgO, and it exemplifies a composition comprising from 62.4 to 69.6 wt % of PbO, from 5.8 to 13.6 wt % of SiO.sub.2, from 13.6 to 20.0 wt % of B.sub.2 O.sub.3, from 0.2 to 1.0 wt % of Al.sub.2 O.sub.3, from 0.0 to 5.0 wt % of MgO and from 0.0 to 6.0 wt % of CaO.
JP-A-53-30269 proposes a low melting point glass having a high lead content and having a small amount of lithium oxide incorporated, and it exemplifies a composition comprising 73 wt % of PbO, 12 wt % of SiO.sub.2, 12 wt % of B.sub.2 O.sub.3, 2.7 wt % of Al.sub.2 O.sub.3 and 0.3 wt % of Li.sub.2 O.
JP-A-60-11246 (EP131389) proposes a glass composition comprising SiO.sub.2, B.sub.2 O.sub.3, ZnO, Al.sub.2 O.sub.3, Na.sub.2 O, K.sub.2 O, CaO and Li.sub.2 O, and it exemplifies a composition comprising 20 wt % of SiO.sub.2, 40 wt % of ZnO, 20 wt % of B.sub.2 O.sub.3, 5 wt % of Al.sub.2 O.sub.3, 7 wt % of Na.sub.2 O, 4 wt % of K.sub.2 O, and 1 wt % of Li.sub.2 O.
JP-A-60-101839 proposes a low melting point glass having lead oxide limited to at most 60 wt % in order to prevent corrosion of thin aluminum electrodes, and it exemplifies a composition comprising from 43 to 50 wt % of PbO, from 8 to 15 wt % of B.sub.2 O.sub.3 from 15 to 30 wt % of SiO.sub.2, and from 5 to 25 wt % of Al.sub.2 O.sub.3.
JP-A-61-110936 proposes a dielectric having a PbO content of from 40 to 50%.
Japanese Patent No. 2503072 proposes a covering made of two layer films, wherein a low melting glass containing, as the main component, PbO having ZnO added thereto, and a low melting point glass containing, as the main component, PbO having Ce.sub.2 O added thereto, are laminated.
JP-A-6-333503 proposes a low melting point glass having lead oxide limited to at most 60 wt % in order to prevent a reaction with a tin oxide type oxide electrode, and it exemplifies a composition comprising 60 wt % of PbO, 10 wt % of B.sub.2 O.sub.3, 25 wt % of SiO.sub.2 and 5 wt % of other inorganic materials.
JP-A-7-176269 proposes to cover with a first dielectric layer formed with a thick film paste containing, as the main component, a low melting point glass, and with a second dielectric layer formed with a thin film paste containing, as the main component, a low melting point glass having a softening point lower than the first dielectric layer and fired at least at a temperature whereby the paste is softened to form a smooth surface layer.
JP-A-7-105855 proposes to cover with dielectric layers which comprise a lower glass layer formed by firing a first glass material at a temperature lower than its softening point, and an upper glass layer formed by firing a second glass material at a temperature higher than its softening point.
JP-A-8-77930 proposes to cover with a dielectric layer composed of a low melting point glass having a dielectric constant of at most 8 and containing Na.sub.2 O and B.sub.2 O.sub.3.
JP-A-9-50769 proposes to cover with a dielectric layer composed of a ZnO type glass material containing substantially no lead.
JP-A-9-102273 proposes to transfer a film-forming material layer containing a glass powder which is a mixture comprising from 60 to 90 wt % of ZnO, from 5 to 20 wt % of B.sub.2 O.sub.3 and from 5 to 20 wt % of SiO.sub.2, to the surface of a glass substrate having electrodes fixed thereon.
As the glass material for a substrate for PDP, soda lime silicate glass is, for example, known, and its average thermal expansion coefficient is from about 80.times.10.sup.-7 to 90.times.10.sup.-7 /.degree.C. within a range of from 50 to 350.degree. C. To form a glass layer on such a substrate, it is believed advisable to employ a low melting point glass having an average thermal expansion coefficient within a range of from 70.times.10.sup.-7 to 85.times.10.sup.-7 /.degree.C. within a range of from 50 to 350.degree. C. in order to secure the strength of the substrate.
To form such an insulating glass layer, it is possible to employ a method of using a vacuum process such as a vapor deposition method or a sputtering method, or a chemical forming method such as a sol-gel method. However, such a method has a problem such that the forming efficiency is poor, thus leading to high costs, or the reliability of the formed layer is low.
Therefore, it is common to employ a method for forming such an insulating glass layer by coating a low melting point glass powder, followed by firing. However, the glass layer thus obtained has low transparency. If the transparency of the glass layer used for the front substrate of PDP is low, it will be impossible to effectively take out plasma-induced fluorescent lights, whereby the display quality is likely to be low.